A SELECTIVE CATALYTIC REDUCTION SYSTEM AND A METHOD FOR NOx REDUCTION

ABSTRACT

A selective catalytic reduction system applying diesel oil as reductant for converting nitrogen oxides by a catalyst into diatomic nitrogen and water in a diesel engine is provided. The selective catalytic reduction system includes an oil injection system, a reactor and a number of selective catalytic reduction catalysts provided in a first section. The selective catalytic reduction system includes at least one additional section including a number of selective catalytic reduction catalysts. The at least one additional section is provided in a non-zero distance from the first section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/DK2019/050075, having a filing date of Feb. 28, 2019, which is based on DK Application No. PA 201800098, having a filing date of Mar. 1, 2018, the entire contents both of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to a selective catalytic reduction system for NOx reduction. The following also relates to a method for NOx reduction. The system and method are configured for selective catalytic reduction of NOx in an exhaust stream either after a four-stroke diesel engine or between the exhaust receiver and the exhaust turbines on a two-stroke diesel engine by using a multifunctioning catalyst with oil as reductant.

BACKGROUND

Selective catalytic reduction systems are used for converting nitrogen oxides, typically referred to as NOx by a catalyst into diatomic nitrogen, N₂ and water, H₂O. The conventional art teaches that a gaseous reductant may be selected among the following: anhydrous ammonia, aqueous ammonia or urea, and may be added to a stream of flue or exhaust gas and be adsorbed onto a catalyst.

Commercial selective catalytic reduction systems are applied in diesel engines, such as those found on ships, diesel locomotives, gas turbines, large utility boilers, industrial boilers, and municipal solid waste boilers and have been shown to effectively reduce NOx.

The increasing environmental awareness regarding NOx emissions has introduced higher requirements for NOx reduction equipment.

It is known that the International Maritime Organization (IMO) gradually has introduced more restrained NOx emission standards. Likewise, NOx emission tax in countries like Norway and Sweden has raised the demand for NOx reduction equipment. Selective Catalytic Reduction systems using urea as reactant followed by Exhaust Gas Recirculation are commonly used.

For the automobile industry, the requirements for the NOx reduction equipment is far more demanding than for the marine industry including NOx reduction at temperatures up 700° C. and here zeolites are more stable than conventional material for catalyst powder typically V₂O₅/WO₃—TiO₂, therefore the interest in using zeolite-based catalysts.

Research has been carried out in order to find a method, in which diesel oil can be used as reductant instead of urea. In 1990, Dr. M. Iwamoto published information about using Cu-ZSM-5 (A Cu ex-changed zeolite catalyst) as catalyst and diesel oil as reductant. (Dr. M. Iwamoto (M Iwamoto, Decomposition of NO on copper ion-exchanged zeolite catalysts, Proceedings of meeting on Catalytic technology for removal of nitrogen monoxide, Tokyo, January 1990, p. 17) found that Cu-ZSM-5 (Copper ion-exchanged zeolites) is fairly active in NO_(x) reduction, if O₂ is present, and hydrocarbons used in the tests were C₂H₄, C₃H₆ and C₃H₈.

In 1996, S. Matsumoto (S. Matsumoto, Toyota, DeNOx catalyst for automotive lean burn engine; Catalysis Today 29 (1996) 43-45) concluded that, “the durability of Cu-ZSM-5 is insufficient for practical use” and introduced the NOx Storage Reduction catalyst (NSR catalyst) consisting of: “precious metals, alkaline earth metals, alumina and some other metal oxides”. Toyota has continued to develop the NSR system now called DiAir System (K. Yoshida et al., Toyota, Development of NSR and DiAir System to Achieve Clean Emission under Transient Cycle; SAE International 2014-01-2809) with pulsating HC injection during desulfation. However, others have continued developing the ZSM-5 catalyst.

It was disclosed (S. A. Yashnik et all, The Cu-ZSM-5 Catalysts Washcoated on Monolith for Diesel Emission Control; Chemistry for Sustainable Development 11 (2003) 309-319), that the Cu-ZSM-5 catalyst has a number of weaknesses, including losses of activity in gas with water and high temperature as well as losses, gradually, of activity with sulphur in the gas. Besides, it might be a problem, that half oxidized carbonous connections stick to the catalyst.

Research has found that, adding of Ce to Cu-ZSM-5 will increase the wet gas activity and hereby prevent formation of CuO, which reduces the activity.

US20070149385A1 discloses a system for reduction of NOx emission from diesel engines using catalysts with diesel oil as reductant. The system applies a multi-functioning catalyst for cracking of diesel oil, NOx reduction and a catalytic partial oxidation material including a platinum-group metal comprises an element selected from the group consisting of rhodium, platinum, iridium, palladium, osmium, and ruthenium to convert coke deposits to hydrogen and carbon monoxide. One disadvantage of this system is, that it requires use of precious metals which are expensive.

Thus, there is a need for an improved selective catalytic reduction system and selective catalytic reduction method for NOx reduction based on application of diesel oil as reductant, wherein a precious metal is not a requirement.

EP1111212A2 describes a selective catalytic NOx reduction system for Diesel engine exhaust applying Diesel oil as reductant. The system comprises a diesel oil injector, a selective catalytic reduction reactor and several selective catalytic reduction sections arranged in series. The catalyst sections each contain several catalyst layers. The distance between adjacent sections are, however, very short. Therefore, the system does not provide an optimum solution for NOx reduction.

EP1893321B1 discloses an NOx reduction system with a first and a second catalytic reactor layers provided in a distance L between each other in the exhaust gas stream. The first and second layers of catalysts may be maintained spaced apart at some distance L, from one another. The conventional art document states, that typically, performance increases as the residence time decreases. According to the understanding of the inventors of the present invention, the opposite is the case: the performance increases as the residence time between the layers increases. However only marginally after a certain residence time.

US2008/053073A1 shows a method and an apparatus for catalytically processing a gas stream passing therethrough to reduce the presence of NOx-Two catalysts are provided with a distance therebetween in a common reactor. It is mentioned that typically performance increases with residence time.

US2010/0251700A1 shows a system and a method for abating NOx emission in an exhaust stream. Selective catalytic reduction catalysts are disclosed.

SUMMARY

An aspect relates to an improved selective catalytic reduction system and selective catalytic reduction method for NOx reduction based on application of diesel oil as reductant, wherein the system and the method reduces or even eliminates the above-mentioned disadvantages of the conventional art.

The selective catalytic reduction system applies diesel oil as reductant for converting nitrogen oxides in diesel engine exhaust, by a catalyst, into diatomic nitrogen (N₂) and water (H₂O). The selective catalytic reduction system comprises:

-   -   an oil injection system adapted to inject diesel oil into the         diesel exhaust;     -   a reactor having a housing provided with an inlet provided in a         first end and an outlet provided in a second end, wherein the         selective catalytic reduction system is adapted to receive         exhaust through the inlet;     -   a number of selective catalytic reduction catalysts provided in         spaced apart sections arranged in series in the housing of the         reactor, which sections are arranged consecutively in a         downstream direction, where each section comprises a plurality         of layers of selective catalytic reduction catalysts provided in         a non-zero distance between one layer of selective catalytic         reduction catalysts and the next layer of selective catalytic         reduction catalysts. According to embodiments of the invention,         the distance between one layer of selective catalytic reduction         catalysts and the next layer of selective catalytic reduction         catalysts is selected to ensure that a minimum average residence         time for the exhaust is provided between each layer, whenever         the oil injection system is active and injecting oil into the         exhaust, wherein the minimum average residence time for moving a         gas molecule from one layer to the next layer is no smaller than         0.025 seconds, or no smaller than 0.04 seconds, and most or no         smaller than 0.135 seconds.

The minimum average residence time is calculated on the basis of an assumption of uniform flow at constant temperature, so that the average residence time may be calculated as the distance between the layers times the cross section and divided by the volumetric flow from one layer to the next. The average residence time may be longer but not shorter than this predefined minimum value of the average residence time.

It is initially assumed that the volumetric flow Q is the cross section times the velocity of the gasses.

Q=A·v  (a)

The average velocity v is calculated as the distance between layers d divided by the average residence time Δt between one layer and the next:

$\begin{matrix} {v = \frac{d}{\Delta t}} & (b) \end{matrix}$

Where d is the distance between one layer and the next, and Δt is the average residence time of gas particles between one layer and the next.

From this, it follows that:

$\begin{matrix} {{\Delta t} = \frac{d \cdot A}{Q}} & (c) \end{matrix}$

When the desired minimum residence time has been determined experimentally, the minimum distance may be calculated as:

$\begin{matrix} {d = \frac{\Delta \; {t \cdot A}}{Q}} & (d) \end{matrix}$

The minimum residence time may be determined by looking at temperature profile in the gasses leaving a layer of catalytic reduction catalysts, as some of the reactions taking place in the space after the layer are exothermic and thus when the temperature does not rise any more, there are no longer any significant processes taking place. Alternatively, the presence or absence of some of the reactants, which are consumed after the catalytic processes in the layer, may be detected, and thus it is possible to determine the duration of half-life of these substances, and thereby reach an estimation on the duration for a predetermined fraction, such as 85%, of these reactants to have undergone a desired reaction. It is also possible to look at the end products in a given reactor, whereby the distances between layers and thus the average residence time is varied, and thus determine when a minimum of the residence time may be reached given a predefined output in terms of pollutants in the gasses which have passed through a reactor.

The temperature of the exhaust entering a first layer will increase compared to the temperature of the exhaust entering the next layer. Accordingly, embodiments of the invention provide a solution, in which the temperature increases downstream the reactor.

Experiments have shown that by choosing non-zero distances defined by the above-mentioned times and defined in the above-mentioned manner, provides a very efficient selective catalytic reduction system.

In one embodiment, the average residence time for moving a gas molecule from one layer to the next layer is at least 0.025 seconds.

In one embodiment, the average residence time for moving a gas molecule from one layer to the next layer is at least 0.04 seconds.

In one embodiment, the average residence time for moving a gas molecule from one layer to the next layer is at least 0.135 seconds.

In an embodiment, at least three layers are provided in the reactor. By this number of layers a desired accumulated cracking rate is easily obtained.

In an embodiment, at least four layers are provided in the reactor.

In an embodiment, at least five layers are provided in the reactor.

In an embodiment, at least six layers are provided in the reactor.

As explained in more detail below, the reaction which takes place in the distance between the layers is the following:

NH₄ ⁺+NO₂⇒N₂+2H₂O;

And here the presence or absence of the reactants NH₄ ⁺ and/or NO₂ may possibly be determined in the hot gasses.

Hereby, it is possible to provide an improved selective catalytic reduction system and selective catalytic reduction method for NOx reduction based on application of diesel oil as reductant with superior performance compared to the conventional art.

Having diesel oil as reactant eliminates the demand for tanks for storing urea on the ship and eliminates potential complex logistics for supplying urea. The cost for oil injected is around the cost for urea injected. On four-stroke medium speed diesel engines around 75% of the energy content of the injected oil can be recovered in a down-stream installed exhaust gas boiler and used for production of water, electricity or heat, meaning the actual operation costs are down to 2% of the oil used for the diesel engine.

By diesel oil is meant a liquid fluid (containing fuel oil) configured to be used in diesel engines. Accordingly, the term diesel oil includes marine diesel oil, also referred to as “distillate marine diesel”, which is a blend of gasoil and heavy fuel oil, with less gasoil than intermediate fuel oil. The term diesel oil includes all fuel oil containing marine fuels.

Furthermore, the use of oil as reactant eliminates the risk of blocking the catalysts due to formation of ammonia-bisulphate (NH₄)HSO₄ or ammonium sulphate (NH₄)₂SO₄.

The reactor of the selective catalytic reduction system according to embodiments of the invention is weighing around 40% less than a conventional urea-SCR reactor with catalysts. Accordingly, embodiments of the invention make it possible to reduce the reactor weight.

The process starts at an exhaust temperature of about 320° C. to about 360° C., such as about 350° C., but can continue operating at an exhaust temperature down to 310° C., because the process causes a temperature increase.

The length of the pipe for reactant injection and evaporation can be as short as one fifth of the urea selective catalytic reduction system pipe and can be made in conventional steel.

Two-stroke diesel engines, due to the temperature increase in the reactor according to embodiments of the invention require a marginal increase in diesel engine specific fuel consumption.

The selective catalytic reduction system according to embodiments of the invention is configured to convert nitrogen oxides (NOx) by a catalyst into diatomic nitrogen (N₂) and water (H₂O) in a diesel engine. The chemical process includes three integrated catalysing steps:

-   -   Cracking of the injected oil;     -   Heterogeneous NOx conversion and formation of radicals used for         a following reaction.     -   Homogeneous NOx conversion downstream of the catalyst.

During the cracking process, the reactant is injected into the exhaust as droplets upstream of the catalyst layer (of the sections) in a manner in which minimum 80% of the droplets have evaporated before reaching the first catalyst layer (of the first section).

The selective catalytic reduction system comprises structures configured to evenly disperse the oil into the exhaust.

The evaporated oil is cracked on the surface of the catalyst layers. A portion of the evaporated oil may, however, continue as un-cracked evaporated oil until it reaches the next catalyst layer. The temperature increases inside the reactor during the process carried out. Due to the increased temperature, only a minimum of the injected oil is passing through the system. The cracking takes place mainly on the surface of the catalyst layer. The cracking on the first layer is expected to be minimum 40%. Accordingly, it is desirable to have several layers provided in a distance from each other. With a cracking rate of 40% at each layer, it is desired to have 3 or more layers.

The diesel oil contains relative long chained hydrocarbon connections. These hydrocarbon connections are cracked to short-chained hydrocarbon connections, during the catalyst process. These hydrocarbon connections are short enough to enter the catalyst layer of the sections. Accordingly, NO is converted to N₂ and H₂O, whereas radicals (NH4⁺) reacts with NO₂ that is converted to N₂ and H₂O. The likely chemical reactions (of the NOx conversion) are shown in the following.

The NO oxidation carried out may be expressed as:

NO+½O₂⇒NO₂  (1)

NO+NO₂+2H⁺⇒2NO++H₂O  (2)

The heterogeneous catalyzing carried out may be expressed as:

2C₄H₈+10O₂+2NO⁺⇒N₂+2CO+6CO₂+8H₂O  (3)

C₃H₆+NO⁺2,5O₂⇒NH₄ ⁺+CO+2CO₂+H₂O  (4)

The homogeneous processes taking place between the layers may be expressed as:

NH₄ ⁺+NO₂⇒N₂+2H₂O  (5)

In an embodiment according to the invention, the selective catalytic reduction system comprises catalyst powder being coated on substrates of honeycomb or corrugated material type. In one embodiment according to the invention, the substrate comprises a ceramic material. In an embodiment according to the invention, the substrate is zeolite. In another embodiment according to the invention, the substrate comprises a metal. In a further embodiment according to the invention, the substrate is a metal. In an even further embodiment according to the invention, the substrate comprises a ceramic material. In another embodiment according to the invention, the substrate is a ceramic material.

The catalyst powder is typically added to the substrate by using a binder. The catalyst powder may comprise a ZSM-n type with n=5, 11 and others as well as modifications thereof. E.g. the copper (Cu) of the ZSM-n type catalyst may be exchanged with either iron (Fe) or magnesium (Mg). In an embodiment according to the invention, the catalyst powder is provided as a coating at least covering the substrate.

In one embodiment, the catalyst powder comprises 1.5-4 wt % Cu.

In one embodiment, the catalyst powder comprises 2.0-3.5 wt % Cu.

Tests have shown that it may be an advantage that the catalyst powder comprises 2.5-3 wt % Cu. In an embodiment, the catalyst powder comprises 2.7-2.9 wt % Cu, such as 2.8 wt % Cu.

In one embodiment according to the invention, the selective catalytic reduction system comprises catalyst powder, where one or more transition metals and one or more stabilizing metals are supported on a zeolite.

It may be beneficial that the zeolite is an alumina silicate.

The zeolite may include ZSM-5, ZSM-11, ZSM-12, Mordenite or Ferrerite.

The transition metal may include one or more of Cu, Fe and Mg.

The transition metal may further include one or more of Ce and Zr as stabilizing metals.

Cu may be added first and Ce and/or Zr may be added after a calcination or Ce and/or Zr may be added in the same operation.

Cu and Ce and/or Zr may be added in the same operation followed by a calcination at a temperature below 550° C.

Catalyst powder may be coated on a substrate of a honeycomb shape or corrugated plate shape, by using of a binder. The substrate may be of a metal type or a ceramic type.

In an embodiment according to the invention, cerium (Ce) or zirconium (Zr), or cerium (Ce) and zirconium (Zr) is used either in one or two sequences optionally with calcination in between the addition in order to provide a more stable (more efficient as function of time) method and reactor used to carry out the method. The binder may comprise up to 50% TiO₂. The Cu content (in the catalyst slurry) may suitably be about 1 wt % to wt 4%, or in the range 1.7 wt % to 3.5 wt %.

In an embodiment according to the invention, the Cu content (in the slurry) is in the range 2.8 wt %-3.1 wt %. The thickness of the coating may be in the range 10-200 μm, or 20-150 μm such as 30-100 μm, e.g. approximately 50 μm.

It may be an advantage that the Ce is present in the catalyst powder in the range 1-5 wt %, or in the range 2.2 wt % to 3.0 wt %, such as about 2.3 wt %. It may be beneficial that the Zr is present in the catalyst powder in the range 1-3 wt %, or about 1.8 wt %.

The amount of carbonized deposits may depend on the sequence of adding supporting metals. Cu may be added first, and Ce added after a calcination. Alternatively, Cu and Ce may be added in the same sequence without calcination in between, reducing the carbonized deposits significantly. Instead of using Cu, it is possible to use iron (Fe) or Mangan (Mg).

It may be an advantage that the selective catalytic reduction system comprises a programmable logic controller.

In an embodiment according to the invention, the amount of injected oil is controlled either by using combined air-oil injection or an “on-off pulse” injection. In case of combined air-oil injection, the air pressure may be adjusted to change the oil amount, as function of the engine load controlled by the programmable logic controller. When the oil is injected in pulses (“on-off”), the amount of injected oil may be controlled by changing the frequency of the pulse. The frequency may be adjusted as function of the engine load controlled by the programmable logic controller. The diesel oil may be injected into a pipe guiding exhaust into the reactor. Said pipe may be arranged before the reactor so that the injection takes place outside the reactor. Diesel oil may, however, also be injected into the reactor as long as the exhaust is capable of evaporating the injected diesel oil before the diesel oil enters the layers of the section(s) inside the reactor.

It may be an advantage that the oil is injected in droplets either by using combined air-oil injection or a pulse injection or a combination thereof.

The programmable logic controller may be configured to control the operation following the diesel engine load and rotational speed. The programmable logic controller may include an integrated safety system enabling the programmable logic controller to generate an alert in case that one or more parameters (e.g. pressure or temperature) no longer are within a predefined range.

It may be an advantage that a hardwired safety system is paralleled to the programmable logic controller to ensure the safety in case of programmable logic controller trips.

The selective catalytic reduction system comprises an oil injection system. The oil injection system is arranged and configured to inject oil into the reactor, hereby enabling the above-mentioned NO oxidation process to be carried out. The oil may be injected below the first catalytic layer (section), or both below the first catalytic layer (section) and between two catalytic layers (section). The oil injection system may have a suitable number of nozzles with a certain size.

It may be an advantage that the selective catalytic reduction system comprises a soot blower arranged to provide air (e.g. with fixed time intervals) under the first catalyst layer in every section in order to keep the catalyst entrance surface clean.

The selective catalytic reduction system comprises a reactor in which the above-mentioned NO oxidation process can be carried out. The reactor is configured to house the sections in order to facilitate the NO oxidation process.

The selective catalytic reduction system comprises a number of selective catalytic reduction catalysts provided in separated layers in a first section, wherein the selective catalytic reduction system comprises at least one additional section comprising a number of selective catalytic reduction catalysts in separated layers, wherein the at least one additional section is provided in a non-zero distance from the first section.

The selective catalytic reduction system may comprise one, two or more sections.

In at least one of the sections, the distance between layers comprising the catalytic reduction system, the specified minimum average residence time is upheld by ensuring that volumetric flow, cross section and distancer between the layers are dimensioned accordingly to reach the desired minimum residence time.

The selective catalytic reduction system may comprise three sections.

The selective catalytic reduction system may be used in a two-stroke diesel engine or a four-stroke diesel engine. It may be advantageous that the selective catalytic reduction system is configured to be installed and used in a two-stroke diesel engine or a four-stroke diesel engine for marine purposes.

It may be an advantage that a non-zero distance is within the range 5-500 mm, or between 10-400 mm, such as 20-250 mm is provided between the layers. Hereby, it is possible to facilitate that the required chemical processes will be carried out as these distances may provide the required minimum average residence time for the gasses given that volumetric flow and cross sections are within boundaries. The non-zero distance may further depend on the thickness of the catalyst layer or catalyst layers as well as on the catalyst or catalysts applied.

It may be an advantage that the selective catalytic reduction system comprises a number of doors moveably attached to the reactor.

Hereby, the sections can be replaced or turned upside down by accessing the sections through the doors.

The doors may be either rotatably attached to the reactor, slidably attached or detachably attached to the reactor or a structure attached thereto.

It may be advantageous that a plate member is provided between at least some of the adjacent sections. The plate member can improve the distribution of oil, hereby providing a more even oil distribution and thus a more effective NOx reduction process.

It may be an advantage that a plate member is provided between the first section and the additional section.

It may be an advantage that a plate member is provided between all adjacent sections.

Hereby, it is possible to provide an even distribution of the oil.

It may be beneficial that the plate member is configured to disperse the oil injected into the interior of the reactor. The plate member may be shaped to guide the evaporated oil radially outwardly to achieve an optimised flow path.

It may be beneficial that the selective catalytic reduction catalysts are arranged in layers each having a thickness within the range 5-200 mm, or 10-150 mm.

In one embodiment, the selective catalytic reduction catalysts are arranged in layers each having a thickness within the range 40-120 mm.

In one embodiment, the selective catalytic reduction catalysts are arranged in layers each having a thickness within the range 50-100 mm.

In one embodiment, the selective catalytic reduction catalysts are arranged in layers each having a thickness within the range 60-90 mm.

It may be an advantage that the reactor comprises one or more sections each comprising several catalyst layers.

It may be advantageous that the selective catalytic reduction catalysts are selected among the following: Ce/Cu-ZSM-5 or Ce—Zr/Cu-ZSM-5, wherein the selective catalytic reduction catalysts are calcinated respectively after Cu adding and after Ce and Zr adding. In one embodiment, the selective catalytic reduction catalysts are dried before being calcinated.

It may be an advantage that the selective catalytic reduction catalysts are selected among the following: Ce/Cu-ZSM-5 or Ce—Zr/Cu-ZSM-5, wherein Cu and Ce and Zr is added at same time and the powder is calcined after the adding of Cu and Ce and Zr.

In one embodiment, Cu, Fe or Mg are added and dried and thereafter Ce and Zr is added and the powder is dried and hereafter calcined.

For both types, TiO₂ may be added to the binder, binding the catalyst powder to the substrate. The substrate consists of either corrugated steel plates or corrugated ceramic plates with 81-256 Cells Per Square Inch (CPSI).

In one embodiment according to the invention, the selective catalytic reduction system comprises an oxidation catalyst. The oxidation catalyst may be arranged after the selective catalytic reduction system in order to oxidize CO to CO₂ and hydro-carbon elements (HC) to CO₂ and H₂O.

It may be an advantage that the selective catalytic reduction system comprises a tubular structure extending centrally along the longitudinal axis of the reactor. Hereby, oil can be injected into the central portion of the reactor in order to provide an optimum distribution of the oil.

The tubular structure extends centrally along the longitudinal axis of the lower conical portion of the reactor.

It may be beneficial that the tubular structure extends through the first section and protrudes therefrom.

The tubular structure may be formed as a pipe, e.g. made of metal or a ceramic material.

It may be advantageous that the selective catalytic reduction system comprises a diffuser. Hereby, the diffuser can be used to optimize the distribution of evaporated oil entering the reactor or oil being injected into the reactor (wherein the injected oil is evaporated inside the reactor).

It may be advantageous that the diffuser is arranged at the inlet portion of the tubular structure. The diffuser may be adapted and arranged to diffuse injected oil towards and onto the first section.

It may be an advantage that the selective catalytic reduction system comprises a heat recovering unit. Hereby, it is possible to recover the heat released in the catalysts. The heat can be used for producing steam that might be used for production of drinking water or electricity to be used onboard.

During the process, the temperature increases over the reactor. Up to 75% of the energy content of the reactant can be recovered because of the increase in exhaust temperature after the reactor.

By way of example, a 2500 kW, four-stroke diesel engine, 720 rpm at 75% load with an exhaust temperature of 340° C. will have around 230 kW-heat extra for steam production, with a better quality (higher degree of superheating). The steam might be used for production of drinking water or electricity.

In one embodiment according to the invention, the system comprises a cooling unit configured to cool down at least a portion of the system. Hereby, it is possible to keep the temperature below a predefined upper temperature limit.

The heat content Q_(oil) in the oil injected can be expressed in a sum of the following components:

Heat loss from the reactor to the ambient: Q_(heat loss)

Heat for increasing inlet temperature to the reactor: Q_(inlet temp increase)

Heat for increasing the temperature over the reactor: Q_(temp over reactor)

Heat loss through the oxidation catalyst: Q_(oxidation)

Accordingly, this can be expressed as:

Q _(oil) =Q _(heat loss) +Q _(inlet temp increase) +Q _(temp over reactor) +Q _(oxidation)

To reduce the ambient Q_(heat loss) the cross section of the reactor may be circular. Accordingly, in an embodiment according to the invention, the reactor comprises a portion that has a circular cross section. It may be an advantage that the reactor comprises a portion that is cylindrical and has a circular cross section.

The method of embodiments of the invention is a method for nitrogen oxides (NOx) reduction, said method comprising the step of using a selective catalytic reduction system and applying diesel oil as reductant for converting nitrogen oxides (NOx) by a catalyst into diatomic nitrogen (N₂) and water (H₂O) in the exhaust of a diesel engine, wherein the method comprises the step of providing diesel oil in a reactor, wherein the method comprises the step of carrying out a NOx conversion including a NO oxidation followed by a heterogeneous catalyzing process followed by a homogeneous process by applying a number of selective catalytic reduction catalysts in a reactor provided in sections arranged in series in the housing of the reactor which sections are arranged consecutively in a downstream direction, where each section comprises a plurality of layers of selective catalytic reduction catalysts. According to the method, the distance between one layer of selective catalytic reduction catalysts and the next layer of selective catalytic reduction catalysts is selected to ensure that a minimum average residence time for the exhaust is provided between each layer, whenever an oil injection system is active and injecting oil into the exhaust wherein the minimum average residence time for moving a gas molecule from one layer of selective catalytic reduction catalysts to a next layer of selective catalytic reduction catalysts is no smaller than 0.025, or no smaller than 0.04, and no smaller than 0.135 seconds.

Hereby, it is possible to carry out an improved selective catalytic reduction process for NOx reduction based on application of diesel oil as reductant, and always ensure optimal function of the reactor.

In order to always reach a desired cracking rate for the used diesel in the exhaust flow, it is desirable to let the exhaust and diesel flow go through at least 3 layers.

It may be beneficial that the method comprises the step of cracking the injected oil, carrying out a heterogeneous NOx conversion that causes formation of the radical NH4⁺, wherein the radical NH4⁺ is used for carrying out homogeneous NO_(x) conversion.

It may be an advantage that the method comprises the step of providing the second section at a non-zero distance from the first section, wherein the non-zero distance is selected in such a manner that the radical NH4⁺ is allowed to react in the above-mentioned chemical process: NH₄ ⁺+NO₂⇒N₂+2H₂O, so that the activity is increased.

It may be advantageous that the method is applied for selective catalytic reduction of NOx in an exhaust stream either after a four-stroke diesel engine or between the exhaust receiver and the exhaust turbine(s) on a two-stroke diesel engine.

The method may be applied for selective catalytic reduction of NOx in diesel engines for marine purposes.

In one embodiment, the selective catalytic reduction system is configured to apply diesel oil as reductant for converting nitrogen oxides (NOx) by a catalyst into diatomic nitrogen (N₂) and water (H₂O) in a diesel engine producing an exhaust gas, wherein the selective catalytic reduction system comprises:

-   -   an oil injection system;     -   a reactor with a number of selective catalytic reduction         catalysts provided in at least a first section, wherein an         oxidation catalyst is arranged to oxidise one or more         compositions in the exhaust gas.

Hereby, it is possible to increase the energy efficiency of the selective catalytic reduction system. A higher heat production can be accomplished. The additional heat can be used to generate electricity (by using a generator driven by a gas turbine and/or a steam turbine) or to evaporate sea-water in order to distillate water (drinking water production).

Furthermore, the selective catalytic reduction system makes it possible to convert all hydrocarbons and particles (soot) contained in the exhaust gas to CO₂, so that an increased heat production can be obtained. It is possible to achieve a heat production that is up to 8% higher than the one achieved by direct oxidation of marine diesel oil. Accordingly, a heat recovery about 100% of the heat value of the oil injected is achievable. This can be achieved because the injected oil is cracked to 80-90% for short-chain hydrocarbons that have a higher heat value than diesel oil. The option of 100% heat recovery enables production of electricity or water production. Therefore, embodiments of the invention enable that the cost of reactant is fully recovered through what is being utilized, nor does it release extra CO₂ by the selective catalytic reduction process.

The selective catalytic reduction system is adapted to receive exhaust through an inlet opening provided in the reactor. The reactor is configured to break down long-chain hydrocarbons into simpler molecules such as light hydrocarbons. The oxidation catalyst is configured to oxidise nitrogen monoxide, NO to nitrogen dioxide, NO₂ through the process: NO+½O₂=NO₂

It is desirable that the particulate filter is configured to continuously oxidize the accumulated soot particulate by NO₂ oxidation. The cracking process carried out in the reactor will cause the temperature to raise above the NO2 soot oxidation temperature. Accordingly, the combustion in the particulate filter will produce additional heat.

Hereby, it is possible to provide an improved selective catalytic reduction system and selective catalytic reduction method for NOx reduction based on application of diesel oil as reductant.

Having diesel oil as reactant eliminates the demand for tanks for storing urea on the ship, and eliminates potential complex logistics for supplying urea. The cost for oil injected is around the cost for urea injected.

By diesel oil is meant a liquid fluid (containing fuel oil) configured to be used in diesel engines. Accordingly, the term diesel oil includes marine diesel oil, also referred to as “distillate marine diesel”, which is a blend of gasoil and heavy fuel oil, with less gasoil than intermediate fuel oil. The term diesel oil includes all fuel oil containing marine fuels.

Furthermore, the use of oil as reactant eliminates the risk of blocking the catalysts due to formation of ammonia-bisulphate (NH₄)HSO₄ or ammonium sulphate (NH₄)₂SO₄.

The reactor of the selective catalytic reduction system according to embodiments of the invention is weighing around 40% less than a conventional urea-SCR reactor with catalysts. Accordingly, embodiments of the invention make it possible to reduce the reactor weight.

The process starts at an exhaust temperature of about 340° C. to about 360° C., such as about 350° C., but can continue operating at an exhaust temperature down to 320° C., because the process causes a temperature increase.

The length of the pipe for reactant injection and evaporation can be as short as one fifth of a conventional urea selective catalytic reduction system pipe and can be made in conventional steel.

The selective catalytic reduction system according to embodiments of the invention is configured to convert nitrogen oxides (NOx) by a catalyst into diatomic nitrogen (N₂) and water (H₂O) in a diesel engine. The chemical process includes three integrated catalysing steps:

-   -   Cracking of the injected oil;     -   Heterogeneous NOx conversion and formation of radicals used for         a following reaction.     -   Homogeneous NOx conversion downstream of the catalyst.

In one embodiment, an oxidation catalyst and a particle filter are arranged after the selective catalytic reduction catalysts.

It may be advantageous that the that the selective catalytic reduction system comprises a flow-through type diesel oxidation catalyst and a soot particle filter (also referred to as a soot particulate filter). Hereby, it is possible to provide a catalyst, that is able to promote oxidation of several exhaust gas components by NO₂. Passing over an oxidation catalyst, the diesel pollutants can be oxidized to harmless products (H₂O and CO₂), and thus can be controlled using the flow through type diesel oxidation catalyst.

The particle oxidation catalyst is a specialized diesel oxidation catalyst with a capacity to hold soot particles, shaped as a device configured to capture and store liquid soot particulate matter material for a period of time sufficient for its catalytic oxidation. The device typically has open flow-through passages that allow exhaust gases to flow, even if the matter material holding capacity is saturated. The captured soot particles must be removed from the device through oxidation to gaseous products, in a process called regeneration. The particle oxidation catalyst regeneration is normally accomplished via reactions between soot and nitrogen dioxide, generated in an upstream oxidation catalyst. A particle oxidation catalyst will not plug once filled with soot to its maximum capacity in the absence of regeneration. Rather, the particulate matter conversion efficiency will gradually decrease, allowing the particulate matter emissions to pass through the structure.

The particle filter may use different types of substrates, and are referred to as flow-through filters, partial flow filters, partial filter technology, open particulate filters, particulate matter filter catalysts and particulate matter oxidation catalysts.

It may be an advantage that the oxidation catalyst is a wall-flow type particulate filter comprising walls coated with a catalyst configured to oxide at least one of the following: CO, HC and NO.

In an embodiment, the oxidation catalyst is a wall-flow type particulate filter comprising walls coated with a catalyst configured to oxide any of the following CO, HC and NO.

The wall-flow type particle filter may be made from ceramic materials such as cordierite, aluminum titanate, mullite, or silicon carbide. The wall-flow type particle filter may comprise a honeycomb structure with alternate channels plugged at opposite ends. Accordingly, while gases pass into the open end of a channel, the plug at the opposite end forces the gases through the porous wall of the honeycomb channel and out through the neighboring channel. The ultrafine porous structure of the channel walls facilitates that a collection efficiency above 85 percent can be achieved. Wall-flow filters capture particulate matter by interception and impaction of the solid particles across the porous wall. The exhaust gas is allowed to pass through in order to maintain low pressure drop.

As a wall-flow type particle filter can fill up over time by developing a layer of retained particles on the inside surface of the porous wall, it is required to provide a burning off or removing accumulated particulate matter and thus regenerating the filter. One suitable disposing of accumulated particulate matter is to continuously oxidize with NO₂ (“passive oxidation”) on the filter when exhaust temperatures are adequate. If the filter temperature is not sufficiently high, the filter temperature is increased by injecting more diesel oil. By burning off trapped material, the filter is cleaned or “regenerated” to its original state. The frequency of regeneration is determined by the amount of soot build-up resulting in an increase in back pressure. To facilitate decomposition of the soot, a catalyst is used in the form of a coating on the filter.

It may be an advantage that the oxidation catalyst is provided between the selective catalytic reduction catalyst and the particulate filter.

It is desirable that the oxidation catalyst is provided downstream the selective catalytic reduction catalyst and the particulate filter

Hereby, it is possible to oxidise nitrogen monoxide, NO to nitrogen dioxide, NO₂ by the oxidation catalyst and use nitrogen dioxide in the particulate filter in order to burn off the accumulated soot particulates.

It may be advantageous that the that the selective catalytic reduction system comprises or is connected to a generator. Accordingly, electricity may be produced.

It may be beneficial that the particulate filter comprises an outlet and that the selective catalytic reduction system comprises or is connected to a generator connected to a turbine, wherein the turbine is in fluid communication with the outlet of the particulate filter.

In one embodiment, the selective catalytic reduction system is connected to or comprises a turbine.

By arranging the turbine in such a manner that the turbine receives the exhaust released from the particulate filter, it is possible to use the exhaust to drive the turbine. Accordingly, the extra energy contained in the exhaust released from the particulate filter will cause rotation of the turbine.

In an embodiment, the turbine comprises a turbine wheel rotatably mounted in a turbine housing. The hot exhaust gas will accumulate in front of the turbine and be converted into kinetic energy in the turbine. Accordingly, the turbine can be accelerated to high speeds. In one embodiment, the exhaust gas flows into the blades of the turbine wheel in the radial direction and then flows out of the turbine wheel in the axial direction.

It may be an advantage that the particulate filter comprises an outlet that comprises or is connected to a distillation apparatus.

Hereby, it is possible to apply the heat generated in the selective catalytic reduction system to produce drinking water.

BRIEF DESCRIPTION

Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein:

FIG. 1 shows a schematic view of a selective catalytic reduction system according to embodiments of the invention;

FIG. 2A shows a schematic view of a selective catalytic reduction system according to embodiments of the invention integrated in a two-stroke diesel engine;

FIG. 2B shows a schematic view of a selective catalytic reduction system according to embodiments of the invention integrated in a four-stroke diesel engine;

FIG. 3A shows a schematic view of a reactor of a selective catalytic reduction system according to embodiments of the invention;

FIG. 3B shows a schematic, cross-sectional view of a reactor of a selective catalytic reduction system according to embodiments of the invention, wherein the reactor comprises a first section and an additional section provided at a non-zero distance from the first section;

FIG. 4A shows a schematic, cross-sectional view of a reactor of a selective catalytic reduction system according to embodiments of the invention, wherein the reactor comprises a first section, a second section provided in a non-zero distance from the first section and a third section provided at a non-zero distance from the second section;

FIG. 4B shows a schematic, cross-sectional view of a portion of a first section and a second section provided at a non-zero distance from the first section of a reactor of a selective catalytic reduction system according to embodiments of the invention;

FIG. 5A shows a top view of a manifold of a selective catalytic reduction system according to embodiments of the invention;

FIG. 5B shows a cross-sectional side view of the manifold shown in FIG. 5A;

FIG. 5C shows a side view of the manifold shown in FIG. 5A;

FIG. 5D shows a perspective view of the manifold shown in FIG. 5A;

FIG. 6A shows a graph showing the relative activity (two sections relative to one section) as function of the residence time between the sections;

FIG. 6B shows a schematic, cross-sectional view of a reactor of a selective catalytic reduction system according to embodiments of the invention, wherein the reactor comprises a first section and an additional section provided at a non-zero distance from the first section;

FIG. 6C shows a perspective, top view of a layer of a section of a reactor according to embodiments of the invention;

Table 1 shows the Cu percentage in slurry, the slurry loading measured in g/L of a various layer configurations;

FIG. 7A shows the catalyst activity as function of the Cu content in the layers described in Table 1;

FIG. 7B is a graph illustrating the NOx reduction degree as function of the Cu content;

FIG. 8 shows 3 somewhat different embodiments in schematic, cross-sectional view of a reactor of a catalytic reduction system according to embodiments of the invention;

FIG. 9 is an enlarged view of a part of the reactor shown in FIG. 8;

FIG. 10A shows a 3D rendering of an embodiment of a cassette with catalysts for insertion in a reactor;

FIG. 10B is a cross sectional view of the cassette seen in FIG. 10A;

FIG. 10C is a side view of the reactor in FIG. 10A;

FIG. 11 is a schematic 3D representation of a cross section of a reactor section with 3 layers;

FIG. 12 shows the cracking process in a reactor;

FIG. 13A shows a schematic side view of a selective catalytic reduction system according to embodiments of the invention;

FIG. 13B shows a selective catalytic reduction system, in which the structures of the reactor are indicated;

FIG. 14A shows a schematic view of a selective catalytic reduction system according to embodiments of the invention configured to carry out NOx reduction and produce steam and power;

FIG. 14B shows a schematic view of a selective catalytic reduction system configured to carry out NOx reduction and produce drinking water and

FIG. 15 shows a schematic view of a selective catalytic reduction system configured to carry out NOx reduction and produce steam and power.

DETAILED DESCRIPTION

Referring now in detail to the drawings for the purpose of illustrating embodiments of the present invention, a selective catalytic reduction system 2 of embodiments of the present invention is illustrated in FIG. 1.

FIG. 1 is a schematic side view of a selective catalytic reduction system 2 according to embodiments of the invention. The selective catalytic reduction system 2 comprises a reactor 10 provided with doors 18, 18′, 18′″ each giving access to a portion of the interior part of the reactor 10. Hereby, it is possible to replace or turn around the sections inside the reactor 10.

The reactor 10 comprises a cylindrical central portion sandwiched between a lower conical portion 20 and an upper conical portion 20′. An end pipe 22 provided with a flange 58 in its distal end is provided in the distal end of the lower conical portion 20. Likewise, an end pipe 22′ provided with a flange 58′ in its distal end is provided in the distal end of the upper conical portion 20′. The doors 18, 18′, 18″ are moveably attached (e.g. rotatably attached or attachably attached) to the central portion of the reactor 10.

It is important to underline that the reactor 10 can be oriented both horizontally and vertically. It may also be possible to arrange the reactor 10 in an inclined orientation so that the reactor is angled relative to both horizontal and vertical.

A pipe 26′ provided with a flange 58 in its proximal end is attached to the end pipe 22′, by fixing the flanges 58, 58′ to each other. Similarly, a bent pipe 26 provided with a flange 58 in its proximal end is attached to the end pipe 22, by fixing the flanges 58, 58′ to each other. It is important to underline that the pipe configuration can be different. In one embodiment according to embodiments of the invention, the pipe 26 may be straight.

An injection unit 24 extends through the wall of the pipe 26 and is configured to inject oil 40 into the pipe 26. The injection unit 24 is configured to inject oil 40 towards the end pipe 22 of the reactor 10. Accordingly, the injected oil 40 will enter the reactor 10, in which the diesel oil is applied as reductant in the selective catalytic reduction system 2.

When oil (in liquid form) is injected into the exhaust in the pipe 26, the diesel oil droplets will evaporate. Accordingly, in an embodiment according to the invention, the system does not require a separate oil evaporation unit.

The selective catalytic reduction system 2 comprises a differential pressure sensor 32 connected to a first portion of the reactor 10 by a first conduit 34 and to a second portion of the reactor 10 by a second conduit 34′. Hereby, the differential pressure sensor 32 is capable of measuring the differential pressure across that part of the reactor 10 that extends between the first portion and the second portion. For practical reasons it may be an advantage that the differential pressure sensor 32 is connected to a lower portion of the reactor 10 by the first conduit 34 and to an upper portion of the reactor 10 by a second conduit 34′. Accordingly, the differential pressure sensor 32 can measure the differential pressure across the central portion of the reactor 10, in which the catalysts are arranged. Therefore, the differential pressure sensor 32 is configured to detect when the differential pressure exceeds a predefined pressure level (e.g. 1-1000 mbar, such as 4-500 mbar, or 8-100 mbar, e.g. 10-20 mbar). The selective catalytic reduction system 2 comprises an alert unit configured to generate an alert when the differential pressure exceeds the predefined pressure level.

In another embodiment according to the invention, two separated pressure sensors are applied to measure the pressure at two different positions of the reactor 10. By comparing the two detected pressures, it is possible to calculate the pressure difference between the two measurement points. Accordingly, the differential pressure sensor 32 may be replaced with two pressure sensors.

The selective catalytic reduction system 2 comprises a first temperature sensor 28 arranged and configured to detect the temperature in the lower (inlet) portion of the reactor 10. The selective catalytic reduction system 2 comprises a second temperature sensor 30 arranged and configured to detect the temperature in the upper (outlet) portion of the reactor 10. By comparing the temperatures detected by the first temperature sensor 28 and the second temperature sensor 30, it is possible to measure the temperature increase across the reactor 10.

The selective catalytic reduction system 2 comprises a pump 48 connected to a diesel tank 50 via a pipe 54″. The pump 48 is a pump configured to generate a sufficiently high pressure. The pump 48 is in fluid communication with the injection unit 24. The pump 48 is connected to a flow sensor 46 through a pipe 54′. A control valve 44 is connected to the flow sensor 46 via a pipe 54 and the control valve 44 is connected to the injection unit 24 via a pipe 36. A pipe 55 extends between the pipe 54′ and the tank diesel tank 50. Accordingly, diesel oil can be returned from the pipe 54′ to the tank diesel tank 50. In one embodiment according to the invention, the selective catalytic reduction system 2 comprises two pumps 48 (a first pump and a second pump) connected in parallel. Hereby, it is possible to apply the second pump in case that the first pump is malfunctioning, needs to be serviced or be replaced, or vice versa.

A tray (for collecting oil) may be arranged under the injection unit 24. Likewise, a tray may be arranged under the pump for collection of leaking oil. The system 2 may comprise a sensor arranged and configured to generate an alert in case of a sufficiently large leakage.

The selective catalytic reduction system 2 comprises a programmable logic controller 60. The differential pressure sensor 32, the temperature sensors 28, 30, pump 48, the flow sensor 46 and the control valve 44 are connected to the programmable logic controller 60 by cables 52, 52′, 52″. However, it is possible to replace this wired connection with a wireless connection (by applying corresponding transmitters and receivers). The programmable logic controller 60 receives the measurements (sensor inputs) detected by the differential pressure sensor 32, the temperature sensors 28, 30, the pump 48, the flow sensor 46 and the control valve 44. The programmable logic controller 60 is configured to control the pump 48 and the control valve 44 on the basis of the sensor inputs of the differential pressure sensor 32, the temperature sensors 28, 30 and the flow sensor 46. The programmable logic controller 60 may be configured to generate an alert when the difference between the detected temperatures exceeds a predefined temperature level or when the difference between the detected temperatures is lower than a predefined temperature level.

FIG. 2A illustrates a schematic view of a selective catalytic reduction system 2 according to embodiments of the invention integrated in a two-stroke diesel engine 4. The two-stroke diesel engine 4 comprises a cylinder 66 with a piston 70. A compressor 80 is arranged to deliver compressed air to a scavenge air receiver 76 that is in fluid communication with the cylinder 66. The exhaust gas leaves the cylinder 66 via a pipe that is connected to an exhaust gas receiver 78. The exhaust gas receiver 78 is connected to an exhaust turbine (expander) 82. The exhaust gas receiver 78 is connected to both the inlet and the outlet of the exhaust turbine 82. A control valve 68′″ is arranged to control the flow through the pipe connecting the gas receiver 78 to the inlet of the exhaust turbine 82. A control valve 68 is arranged to regulate the flow from a pipe leaving the gas receiver 78.

The selective catalytic reduction system 2 comprises an oil pump 48, an oil injector 6 and a reactor 10. The pump 48 is arranged to deliver pressurised oil to the oil injector 6. The oil injector 6 is arranged and configured to inject pressurised oil into the reactor 10. A blower 72 is arranged to blow (compressed) air into the reactor 10. In an embodiment according to the invention, the blower 72 is configured to blow air under the first catalyst layer in every section of the catalyst in a predefined manner, with fixed time intervals to keep the catalyst entrance surface clean.

The reactor 10 is arranged between the outlet of the exhaust gas receiver 78 and the exhaust turbine 82. An oil injection system 6 is arranged between the exhaust gas receiver 78 and the reactor 10.

The selective catalytic reduction system 2 comprises a control valve 68′ arranged between the exhaust gas receiver 78 and the oil injection system 6.

In one embodiment according to the invention, the blower 72 may be an integrated part of the selective catalytic reduction system 2.

In another embodiment according to the invention, the blower 72 may be a separate unit not being an integrated part of the selective catalytic reduction system 2.

A control valve 68″ is arranged after the outlet of the reactor 10. The control valve 68′ is configured to regulate the flow from the reactor 10.

The scavenge air receiver 76 is connected to the exhaust turbine 82. A control valve 74 is provided between the scavenge air receiver 76 and the exhaust turbine 82. The control valve 74 may be of any suitable type and size.

The selective catalytic reduction system 2 comprises a control unit 62 configured to control a number of units of the selective catalytic reduction system 2. In one embodiment according to the invention, the selective catalytic reduction system 2 comprises a control unit 62 configured to control the pump 48, and at least a selection of the control valves 68, 68′, 68″, 68′″ and the brake resistor 74. In an embodiment according to the invention, the selective catalytic reduction system 2 comprises a control unit 62 configured to control the pump 48, the control valves 68, 68′, 68″, 68′″ and the brake resistor 74 and the oil injection system 6.

The selective catalytic reduction system 2 may comprise a number of sensors (e.g. as shown in FIG. 1) and an alert unit configured to generate an alert when one or more parameters detected by one or more of the sensors exceed a predefined level or is smaller than a predefined level.

As shown in FIG. 2A, the selective catalytic reduction system 2 is configured for selective catalytic reduction of NOx between the exhaust receiver 78 and the exhaust turbine 82 on a two-stroke diesel engine 2 (e.g. for marine purpose).

FIG. 2B illustrates a schematic view of a selective catalytic reduction system 2 according to embodiments of the invention integrated in a four-stroke diesel engine 4. The four-stroke diesel engine 4 is symbolised by a cylinder 66 with a piston 70. A compressor 80 is arranged to deliver compressed air to an air cooler 30 being in fluid communication with the cylinder 66.

The exhaust gas leaves the cylinder 66 via a pipe that is connected to an exhaust gas receiver 78. The exhaust gas receiver 78 is connected to an exhaust turbine 82.

The reactor 10 is arranged after the exhaust turbine 82. The selective catalytic reduction system 2 comprises an oil injector 6 and a reactor 10. A diesel tank 50 is in fluid communication with the oil injector 6. Accordingly, the tank 50 is configured to deliver oil to the oil injector 6. The exhaust turbine 82 is connected to the oil injector 6, which is arranged and configured to inject pressurised oil into the reactor 10. A blower 72 is arranged to blow (compressed) air into the reactor 10. In an embodiment according to the invention, the blower 72 is configured to blow air under the first catalyst layer in every section of the catalyst in a predefined manner, with fixed time intervals to keep the catalyst entrance surface clean.

In one embodiment according to the invention, the blower 72 may be an integrated part of the selective catalytic reduction system 2.

In another embodiment according to the invention, the blower 72 may be a separate unit not being an integrated part of the selective catalytic reduction system 2.

The selective catalytic reduction system 2 comprises a control unit 62 configured to control a number of units of the selective catalytic reduction system 2. In one embodiment according to the invention, the selective catalytic reduction system 2 comprises a control unit 62 configured to control the oil injection system 6 as well as one or more structures of the selective catalytic reduction system 2.

The selective catalytic reduction system 2 may comprise one or more control valves (not shown) and the control unit 62 may be configured to control one or more of these control valves. In one embodiment according to the invention, the control unit 62 is configured to receive one or more signals from one or more sensors and to regulate one or more devices on the basis of the received signal(s). The control unit 62 may be configured to control one or more valves and/or the oil injection system 6 on the basis of temperature and/or differential pressure detections.

The selective catalytic reduction system 2 may comprise a number of sensors (e.g. as shown in FIG. 1) and an alert unit configured to generate an alert when one or more parameters detected by one or more of the sensors exceed a predefined level or is smaller than a predefined level. FIG. 3A illustrates a schematic view of a reactor 10 of a selective catalytic reduction system according to embodiments of the invention. The reactor 10 comprises a cylindrical central portion provided with a first door 18 arranged in the lower end (with respect to the flow direction indicated by an arrow) of the central portion and a second door 18 arranged in the upper end (with respect to the flow direction indicated by an arrow) of the central portion. The doors 18, 18′ are configured to be opened or removed in order to access the interior of the reactor 10. This enables an easy replacement of the layers (see FIG. 3B) arranged inside the reactor 10.

The central portion is sandwiched between a lower conical portion 20 and an upper conical portion 20′. An end pipe 22 provided with a flange 58′ in its distal end is provided in the distal end of the lower conical portion 20. Likewise, an end pipe 22′ provided with a flange 58′ in its distal end is provided in the distal end of the upper conical portion 20′.

A bent pipe 26 provided with a flange 58 in its proximal end is attached to the end pipe 22, by attachment of the adjacent flanges 58, 58′ to each other. The pipe 26 may have another configuration. The pipe 26 may by way of example be straight.

A pressure tank 31 configured to deliver pressurised air is connected to the reactor 10 by a first conduit and a second conduit. A valve 33 is arranged in the first conduit between the pressure tank 31 and the reactor 10, whereas another valve 33 is arranged in the second conduit between the pressure tank 31 and the reactor 10.

The reactor 10 is provided with a first support leg 90, a second support leg 90′ and a third support leg (not shown). The reactor 10 may be applied in a selective catalytic reduction system that comprises a control unit configured to control the one or more oil injectors and/or one or more control valves (not shown). The control unit may be configured to receive information from one or more sensors including the differential pressure sensor 32.

FIG. 3B illustrates a schematic, cross-sectional view of a reactor 10 of a selective catalytic reduction system according to embodiments of the invention. The reactor 10 basically corresponds to the one shown in FIG. 3A. The reactor 10 comprises a cylindrical central portion that is sandwiched between a lower conical portion 20 and an upper conical portion 20′. An end pipe 22 provided with a flange 58′ in its distal end is provided in the distal end of the lower conical portion 20. Likewise, an end pipe 22′ provided with a flange 58′ in its distal end is provided in the distal end of the upper conical portion 20′.

Inside the interior of the cylindrical central portion a first section 14 and an additional section 16 are provided in a non-zero distance D₃ from the first section 14. A plate member is provided between the first section 14 and the additional section 16. The plate member 84 is configured to disperse the oil injected into the interior of the reactor 10.

Each section 14, 16 comprises several layers S₁, S₂, S₃ and S₁′, S₂′, S₃′, respectively. The distance between adjacent layers S₁, S₂, S₃, S₁′, S₂′, S₃′ and the distance D₃ between the first section 14 and the additional section 16 provides time for the generated free radical NH₄ ⁺ to have an impact through its reaction during the previously mentioned homogeneous catalyzing step:

NH₄ ⁺+NO₂⇒N₂+2H₂O  (5)

Accordingly, by providing a non-zero distance between adjacent layers S₁, S₂, S₃, S₁′, S₂′, S₃′ and between the first section 14 and the additional section 16, it is possible to increase the effect and efficiently of the reactor 10. The distance between adjacent layers S₁, S₂, S₃, S₁′, S₂′, S₃′ and the distance D₃ between the first section 14 and the additional section 16 is selected in such a manner that maximum effect of the free radical NH₄ ⁺ can be achieved. The distance D₃ is between 5-1000 mm, or in the range 50-500 mm, such as 100-400 mm.

A diffuser 88 is provided in the lower conical portion 20. The diffuser is configured to mix and diffuse the injected oil 40 towards and onto the first layer S₁ of the first section 14.

The exhaust to be treated enters the reactor 10 through the inlet provided in the first end 92, passes the first section 14, the second section 16 and leaves the reactor 10 through the outlet provided in the second end 94. The exhaust carries the injected oil 40 in gaseous form. The gaseous oil 40 is initially guided outwardly by the diffuser 88. A closing structure 134, 134′, 134″ extends radially in extension of each layer S₁, S₂, S₃. Accordingly, the closing structures 134, 134′, 134″ force the evaporated diesel oil 40 to pass (axially) through the passages provided in the layer S₁, S₂, S₃.

The first section 14 comprises three layers S₁, S₂, S₃ arranged above each other separated by a gap. Likewise, the second section 16 comprises three layers S₁′, S₂′, S₃′ arranged above each other, wherein a distance is provided between adjacent layers S₁′, S₂′, S₃′.

The layers S₁, S₂, S₃ of the first section 14 and the layers S₁′, S₂′, S₃′ of the second section 16 may form a Ce/Cu-ZSM-5 type catalyst or a Ce—Zr/Cu-ZSM-5 type catalyst.

When a Ce/Cu-ZSM-5 type catalyst is used, the catalyst may be calcinated respectively after Cu adding and after Ce and Zr adding.

When a Ce—Zr/Cu-ZSM-5 type catalyst is used, Cu and Ce and Zr may be added at same time and powder may be calcined after the adding of Cu and Ce and Zr.

For both types, TiO₂ may be added to the binder, binding the catalyst powder to the substrate. The substrate may consist of either corrugated steel plates or corrugated ceramic plates with a CPSI (Cells Per Square Inch) in the range 81-256.

The layers S₁, S₂, S₃ of the first section 14 and the layers S₁′, S₂′, S₃′ of the second section 16 may have essentially the same thickness. The thickness of each layer may be 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm.

In an embodiment according to the invention, the layer S₁ has a thickness of 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm. In an embodiment according to the invention, the layer S₂ has a thickness of 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm. In an embodiment according to the invention, the layer S₃ has a thickness of 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm.

In an embodiment according to the invention, the layers S₁′, S₂′ and S₃′ have a thickness of 5-500 mm, or 10-250 mm such as 40-150 mm, e.g. 75 mm.

In an embodiment according to the invention, the layers S₁, S₂, S₃ are of equal type and dimension.

In another embodiment according to the invention, the layers S₁, S₂, S₃ are of different equal type and/or thickness.

A plate member 84 is arranged between the first section 14 and the second section 16. A tubular structure 86 formed as a centrally arranged tube extends along the longitudinal axis of the lower conical portion 20 of the reactor 10. Evaporated oil 40′ is introduced into the space between the two sections 14, 16. The distal end of the tubular structure 86 is provided in a distance h to the plate member 84. The plate member 84 is configured and arranged to guide the evaporated diesel oil 40′ radially towards the periphery of the reactor 10, from which the evaporated diesel oil 40′ is guided towards the first layer S₁′ of the second section 16.

The selective catalytic reduction system according to embodiments of the invention may comprise an additional oxidation catalyst reactor (not shown) configured to oxidize CO to CO₂ and HC to CO₂ and H₂O. The additional oxidation catalyst reactor may be arranged after the reactor 10.

Under the first catalyst layer S₁ (arranged closest to the inlet 92 of the reactor 10) in the first section 14 and the first catalyst layer S₁′ in the second section 16, air may be blown with fixed time intervals to keep the catalyst entrance surface clean. This may be done by a blower (as illustrated in FIG. 2A and FIG. 2B).

The catalysts of the sections 14, 16 ensure three integrated catalyzing processes including:

-   -   Cracking of oil;     -   Heterogeneous NOx conversion and formation of radicals used for         a following reaction;     -   Homogeneous NOx conversion downstream the catalyst.

The temperature increases across the first section 14. This temperature increase ΔT₁ is indicated in FIG. 3B. Likewise, the temperature increases across the second section 16. This temperature increase ΔT₂ is also indicated in FIG. 3B.

In one embodiment according to the invention, the selective catalytic reduction system comprises a waste heat recovering system arranged after the reactor 10. Hereby, the waste heat recovering system can recover the heat released in the catalysts of the reactor 10.

The recovered heat may be used for producing steam for production of drinking water or electricity (that may be used onboard if the selective catalytic reduction system is applied for selective catalytic reduction of NOx in an exhaust stream in a marine diesel engine).

FIG. 4A illustrates a schematic, cross-sectional view of a reactor 10 of a selective catalytic reduction system according to embodiments of the invention. The selective catalytic reduction system comprises a reactor 10 provided with a first section 14, a second section 16 provided in a non-zero distance D₁ from the first section 14 and a third section 16′ provided in a non-zero distance D₂ from the second section 16. The reactor 10 basically corresponds to the one shown in FIG. 3A and FIG. 3B. The reactor 10 comprises a cylindrical central portion that is sandwiched between a lower conical portion 20 and an upper conical portion 20′. An end pipe 22 provided with a flange 58′ in its distal end, is provided in the distal end of the lower conical portion 20. Likewise, an end pipe 22′ provided with a flange 58′ in its distal end is provided in the distal end of the upper conical portion 20′.

Inside the interior of the cylindrical central portion, however, a first section 14 and an additional section 16 are provided in a non-zero distance D₁ from the first section 14. A second plate member 84′ is provided in a non-zero distance D₂ between the additional (second) section 16 and the third section 16′. The plate members 84, 84′ are configured to disperse the oil 40′ injected into the interior of the reactor 10.

The distance D₁ between the first section 14 and the additional section 16 as well as the distance between adjacent layers S₁, S₂, S₃, S₁′, S₂′, S₃′ provides time for the generated free radical NH₄′ to have an impact through its reaction with nitrogen dioxide (NO₂) forming diatomic nitrogen gas (N₂) and water (H₂O). Likewise, the distance D₂ between the additional section 16 and the third section 16′ as well as the distance between adjacent layers S₁″, S₂″, S₃″ provides time for the generated free radical NH₄ ⁺ to have an impact through the above-mentioned reaction with nitrogen dioxide (NO₂).

By providing a non-zero distance D₁, D₂ between the adjacent sections 14, 16, 16′ and between adjacent layers S₁, S₂, S₃, S₁′, S₂′, S₃′, S₁″, S₂″, S₃″, it is possible to improve the effect and efficiently of the reactor 10. Said distances may be selected in such a manner that maximum effect of the free radical NH₄ ⁺ can be achieved. The distances D₁, D₂ may be between 5-1000 mm, or in the range 50-500 mm, such as 100-400 mm. In one embodiment according to the invention, the distances D₁, D₂ are equal. The distance between adjacent layers S₁, S₂, S₃, S₁′, S₂′, S₃′, S₁″, S₂″, S₃″ may be between 5-1000 mm, or in the range 25-500 mm, such as 50-400 mm.

A tubular structure 86 extends centrally along the longitudinal axis of the lower conical portion 20 of the reactor 10. The tubular structure 86 extends through the first section 14 and protrudes therefrom. A diffuser 88 is arranged at the inlet portion of the tubular structure 86. The diffuser 88 is adapted and arranged to mix and diffuse injected oil towards and onto the first layer C₁ of the first section 14.

Exhaust from the engine in which the selective catalytic reduction system is installed, enters the reactor 10 through the inlet provided in the first end 92, passes the first section 14, the second section 16, the third section 16′ and leaves the reactor 10 through the outlet provided in the second end 94.

The first section 14 comprises three layers S₁, S₂, S₃ arranged above each other, wherein adjacent layers are axially spaced from each other. The second section 16 comprises several layers S₁′, S₂′, S₃′ arranged above each other and are spaced from each other. The third section 16′ comprises several layers S₁″, S₂″, S₃″ spaced from each other.

The sections 14, 16, 16′ may be of the same type as the sections explained with reference to FIG. 3B. The sections 14, 16, 16′ may have the same geometry (including thickness) as the sections explained with reference to FIG. 3B.

The temperature increases across all sections 14, 16, 16′. The temperature increase ΔT₃, ΔT₄, ΔT₅ across each of the segments 14, 16, 16′, is indicated in FIG. 4A.

In one embodiment according to the invention, the selective catalytic reduction system comprises a waste heat recovering system (not shown) arranged after the reactor 10. Hereby, the waste heat recovering system can recover the heat released in the catalysts of the reactor 10. Accordingly, the recovered heat may be used for producing steam for production of drinking water or electricity (that may be used onboard if the selective catalytic reduction system is applied for selective catalytic reduction of NOx in an exhaust stream in a marine diesel engine).

FIG. 4B illustrates a schematic, close-up view of a portion of a first layer S₁ and a second layer S₂ provided in a non-zero distance from the first layer S₁. The adjacent layers S₁, S₂ may be layers of the same section of adjacent sections of a reactor of a selective catalytic reduction system according to embodiments of the invention. Each layer S₁, S₂ comprises a structure provided with a plurality of passages 112, 112′ extending through the layer S₁, S₂. Each layer S₁, S₂ comprises a substrate 108, 108′ at least partly, completely covered by a catalyst powder 109, 109′ attached to the substrate as a coating.

The cracking process represents the first process step inside the reactor. The diesel oil used as reactant is injected into the exhaust as small droplets upstream (in the direction indicated by the arrow) the catalyst layer of the first layer S₁. The injection of oil is carried out in such a manner that minimum 80% of the oil droplets are evaporated before reaching the first catalyst layer S₁. The selective catalytic reduction system is configured to evenly disperse the oil into the exhaust.

The evaporated oil is cracked on the surface of the catalyst layer S₁. Accordingly, cracked oil 110 will be present on the distal portion of the substrate 108, 108′. A portion of the oil may flow un-cracked towards the passages 112, 112′ of the first catalyst layer S₁ and the second catalyst layer S₂. Due to the increased temperature (as explained with reference to FIG. 4A), only a minimum of the injected oil is passing through the system.

When the selective catalytic reduction system is used in marine engines, the diesel oil contains relative long-chained hydrocarbon (HC) connections. These connections are cracked to short-chained HC connections during the catalyst process. Accordingly, the short-chained HC connections will be short enough to enter the catalyst layer and convert NO to N₂ and H₂O and further produce radicals (NH4⁺). The chemical reactions (of the NOx conversion) are show in the following.

The NO oxidation carried out may be expressed as:

NO+½O₂⇒NO₂  (1)

NO+NO₂+2H⁺⇒2NO++H₂O  (2)

The heterogeneous catalyzing carried out may be expressed as:

2C₄H₈+10O₂+2NO⁺⇒N₂+2CO+6CO₂+8H₂O  (3)

C₃H₆+NO⁺2,5O₂⇒NH₄ ⁺CO+2CO₂+H₂O  (4)

The homogeneous catalyzing carried out may be expressed as:

NH₄ ⁺+NO₂⇒N₂+2H₂O  (5)

FIG. 5A illustrates a top view of a manifold 100 of a selective catalytic reduction system according to embodiments of the invention. FIG. 5B illustrates a cross-sectional side view of the manifold 100 shown in FIG. 5A. FIG. 5C illustrates a side view of the manifold 100 shown in FIG. 5A and FIG. 5D a perspective view of the manifold 100 shown in FIG. 5A.

The manifold 100 comprises a cylindrical tubular body portion that is provided with a first flange 102 in the inlet end of the manifold 100 and a second flange 104 in the opposite outlet end of the manifold 100. The flanges 102, 104 are configured to be attached to an adjacent pipe (not shown) provided with a matching flange.

The manifold 100 comprises four connection pipes 96, 96′, 96″, 96′″ evenly distributed along the circumference of the manifold 100. The connection pipes 96, 96′, 96″, 96′″ are angled relative to the longitudinal axis of the cylindrical tubular body portion.

Each connection pipes 96, 96′, 96″, 96′″ is provided with a flange 98 in its distal end. A nozzle 106 extends through the interior of each connection pipes 96, 96′, 96″, 96′″. The nozzles 106 are arranged and configured to inject oil into the upper, central portion of the manifold 100.

FIG. 6A illustrates two graphs each showing the relative activity 118 (two sections relative to one section) as function of the residence time 116 (measured in seconds) between the sections for the homogeneous NOx reduction process as indicated in the above-mentioned equation (5).

FIG. 6A illustrates two examples of how the NOx reduction activity 118 varies as function of distance between two layers (shown as the time 116 for a gas particle to move from one section to the other section). When one layer is placed in a very small distance from the adjacent layer, the increase in activity 118 is marginal, whereas when the distance is increased, the activity is increased. The activity 118, however, depends on the catalyst specification (the layers making up the section).

The uppermost graph comprises points 122 representing the activity when two layers of different types are applied. The activity is measured relative to a reference activity in a reactor comprising only a single layer.

The lowermost graph comprises points 120 representing the activity when the catalyst layer thickness is low. The activity is measured relative to the activity in a reactor comprising only a single section.

It can be seen that the relative activity 118 is very low compared to the situation wherein the catalyst layer thickness is larger. Both graphs increase as function of residence time 116. Accordingly, it is possible to achieve an increased efficiency of the selective catalytic reduction system according to embodiments of the invention by introducing two sections being provided in a non-zero distance from each other.

The lowermost graph, however, illustrates that the effect of several layers is minimal when the catalyst layer is thin. Accordingly, it is desirable that the catalyst layer exceeds a predefined minimum layer thickness. The predefined minimum catalyst layer thickness may depend on the relative content of Cu, Fe or Mg.

FIG. 6B illustrates a schematic, cross-sectional view of a reactor 10 of a selective catalytic reduction system according to embodiments of the invention, wherein the reactor 10 comprises a first section 14 and an additional section 16 provided at a non-zero distance D₃ from the first section 14. The reactor 10 basically corresponds to the one shown in FIG. 3B. The reactor 10, however, does not comprise a tubular structure.

FIG. 6C illustrates a perspective, top view of a layer S₁ of a section of a reactor according to embodiments of the invention. The layer S₁ is provided with a plurality of longitudinal extending passages 112. The passages 112 have rectangular, or square, cross sections, of equal size. Each passage 112 is, at least partly, covered with a catalyst coating. In an embodiment (not show), corrugated plates are applied. In this embodiment, the passages are not rectangular.

The height H of the layer S₁ may be in the range 20-300, such as 40-150 mm, e.g. 60-120 mm.

Table 1 illustrates the Cu percentage in slurry, the slurry loading measured in g/L of a first layer configuration called 1A-1B, a second layer configuration called 1Bb and a third layer configuration called 1B-1C. In the first layer configuration, 1A-1B, the layers are arranged on the top of each other in the two sections. In the second layer configuration, 1Bb, there is no space between the layers in two sections, whereas the third layer configuration, 1B-1C, a space between adjacent layers.

FIG. 7A illustrates the catalyst activity 126 (NOx reduction measured in ppm) as function of the Cu content in the layers described in Table 1. FIG. 7A depicts the NOx reduction as function of the number of layers 124. A layer, 1B, arranged under predefined reference conditions is indicated with open squares. The first layer configuration, 1A-1B is indicated with solid circles, the second layer configuration 1Bb is indicated with open circles, whereas the third layer configuration, 1B-1C is indicated with open triangles.

FIG. 7A illustrates that 1, the layers in the first layer configurations 1A-2B having a 1.7% Cu content, can be placed on top of each other. This, however, provides a relative low activity 126. Increased Cu loading increases the activity 16, however, also the production of radical's, explained with reference to the previously mentioned equation (4) and (5), which has to enter a homogeneous reduction process after the catalyst not inside the catalyst.

In FIG. 7A, the layers in the second layer configuration, 1Bb are placed on top of each other in two adjacent sections. FIG. 7A shows, that with the relative high Cu content (2.9%) there is no outcome of adding an extra layer on top of the first layer in each section.

Type 1B and 1C are placed in one section with space in between. An increase in activity due to the homogeneous catalysing process after the catalyst can be observed.

FIG. 7B is a graph illustrating the NOx reduction degree as function of the Cu content. It can be seen that the highest NOx reduction degree is found for a CU content of approximately 2.8%. Accordingly, in an embodiment, the catalyst powder comprises 2.7-2.9 wt % Cu, such as 2.8 wt % Cu.

FIG. 8 discloses 3 different reactors 10, where the reactor far left comprises five sections 14, 16, 16′, 16″ and 16′″ separated by distances D₁, D₂, D₄ and D₄. The reactor depicted centrally in FIG. 8 has 4 sections, where the reactor at the far-right side of the figure has 3 sections. In each section in the reactors shown in FIG. 8 there are 5 spaced apart layers S₁, S₂, S₃, S₄, S₅. (See FIG. 9). In other respects, the reactors shown in FIG. 8 are similar to the reactors depicted in FIGS. 3B and 4A.

In FIG. 9 an enlarged view of a section 16 with accompanying distance D₁ is provided. Layers S₁-S₅ are disclosed and the distances d₁-d₄ between the layers are also indicated. As seen the distance D₁ is significantly larger than each one of the thicknesses L₁-L₅ of the layers S₁-S₅. This corresponds to the distance relations previously mentioned. The distances d₁-d₄ indicated between the layers, may be in the order of the thickness L₁-L₅ of the individual layers.

In the embodiment shown in FIG. 9 an arrow w indicates a movement which the diffuser 88 may perform in order to get closer to or further away from the first section 14 in a reactor 10. By adjusting this distance correctly, the time allowed between injection of oil 40 and the contact between oil and the first layer of the first section may be either reduced or diminished.

FIGS. 10A, 10B an 10C shows an embodiment wherein the layers E₁ and E₂ are shaped differently than in the previously mentioned embodiments. Each of the layers E₁ and E₂ are basically made from the same material as previously mentioned layers S₁-S₅; S₁′-S₅′; S₁″-S₅″ and are also structured with through going canals running straight through the thickness from one side to an opposed side. However, as seen in FIG. 10A, each of the layers E₁, E₂ are composed of square blocks, which are assembled in a 2×2 flat upper layer E₁, and a 2×2 lower layer E₂. Upper layer E₁ and lower layer E₂ are assembled in a nearly cubic shaped cassette 140. FIG. 10B shows a sectional view of a cubic shaped cassette along line A-A in FIG. 10C.

An example of measures for a cassette adapted for a specific catalytic reduction system is provided in FIG. 10C and FIG. 10B. As can be seed in FIG. 10B a distance of 150 is donated between the layers E₁ and E₂, whereas each of the layers has a thickness of 75. The nearly cubic cassette has a height of 302 and a measure 307 along each side. The provided measures are given in mm; but could well be given in inches or scaled with another scaling size according to the use of the cubic cassette. The cassette 140 is particularly well suited for systems where regular exchange of the cassettes are foreseen, and where shipment of cassettes from stock to place of use will frequently be undertaken.

In the cross section of a part of a section of a reactor disclosed in 3D view in FIG. 11, the flow Q is shown by arrow Wq. It is initially assumed that the volumetric flow Q is the cross section times the velocity of the gasses. This is a fair assumption between layers. The thickness of an individual layer is marked D. The distance between the layers is marked d. During passage of gasses we have the following relationship between volumetric flow through the space d between the layers:

Q=A·v  (1)

Here A is the cross-sectional area through which the gas flows and perpendicular to the gas flow direction. The average velocity v is calculated as the distance between layers d divided by the average residence time Δt between one layer and the next:

$\begin{matrix} {v = \frac{d}{\Delta t}} & (2) \end{matrix}$

Where d is the distance between one layer and the next, and Δt is the average residence time of gas particles between one layer and the next. It is here assumed, that the gas flow is uniform across the cross section of the reactor, and that there are no significant changes in temperature during the gas passage over the distance d. This may not be entirely true in real life and a minor compensation for rising temperature and thus increase volumetric gas flow may have to be made.

From the above, it follows that:

$\begin{matrix} {{\Delta t} = \frac{d \cdot A}{Q}} & (3) \end{matrix}$

When the desired minimum residence time has been determined experimentally, the minimum distance may be calculated as:

$\begin{matrix} {d = \frac{\Delta \; {t \cdot A}}{Q}} & (4) \end{matrix}$

Experiments have shown that by choosing non-zero distances defined by the above-mentioned times and defined in the above-mentioned manner, provides a very efficient selective catalytic reduction system.

In example 1, d is chosen to be 40 mm, and with area A of 0.78 m² and Q at 921 Nm³/h a residence time of 0.04 seconds are arrived at.

In example 2, the distance d is 110 mm which yields a residence time of 0.135 seconds given the same area A and flow Q as in example one.

In FIG. 12 a sequence of layers within a reactor is shown. Above each layer there is an indication of the fraction of the added diesel which will be cracked in each layer. In case that each layer cracks a fraction G of the remaining un-cracked oil in the gasses, the following equation gives the total cracking:

G _(total) =G+G(1−G)+G(1−(G(1−G)))+ . . .   (5)

If a cracking degree of a minimum size is desired, and the cracking fraction in each layer is known, this will then demand a given number N of layers. N may easily be determined, once the property of the individual layers are known.

It has been determined, that with the layers as indicated above, a minimum number of 3 layers will provide the desired cracking. In one embodiment, the reactor comprises four or more layers. In an embodiment, the reactor comprises five or more layers. In an embodiment, the reactor comprises six or more layers.

FIG. 13A is a schematic side view of the reactor 10 of a selective catalytic reduction system 2 according to embodiments of the invention. FIG. 13B illustrates the reactor 10 of a selective catalytic reduction system 2, in which the structures of the reactor 10 are indicated.

The selective catalytic reduction system 2 comprises a reactor 10, an oxidation catalyst 136 and a particulate filter 138. The reactor 10 is provided with an opening in its distal end, wherein the oxidation catalyst 136 is arranged in the proximal end of the reactor 10. The particulate filter 138 is arranged in the distal end of the oxidation catalyst 136 above the oxidation catalyst 136.

The selective catalytic reduction system 2 is configured to receive exhaust through the opening in the reactor 10. The reactor 10 is configured to break down long-chain hydrocarbons into simpler molecules such as light hydrocarbons. The reactions carried out in the reactor 10 will be explained in further detail in the following.

The oxidation catalyst 136 is configured to oxidise nitrogen monoxide, NO to nitrogen dioxide, NO₂ through a process as expressed in the following:

NO+½O₂=NO₂

Wall-flow type particulate filters usually remove 85-100% of the soot. In an embodiment, the particulate filter 138 is designed to burn off the accumulated particulate either. This can be accomplished by heating the particulate filter 138 to soot combustion temperatures. The NOx reduction process carried out in the reactor 10 causes the temperature to raise to the required level. Thus, the combustion process in the particulate filter 138 will cause production of additional heat.

Accordingly, the selective catalytic reduction system 2 is capable of producing an enlarge quantity of heat compared to conventional art selective catalytic reduction systems. The heat may be applied to produce electricity by using a turbine (see FIG. 14A) and/or to produce drinking water by using a distillation apparatus (see FIG. 14B).

The reactor 10 makes it possible to break down long-chain hydrocarbons into simpler molecules such as light hydrocarbons. Hereby, an increased heat production can be achieved.

The exhaust enters from the bottom side of the reactor 10. Diesel oil is injected into the reactor 10. The oil is evaporated due to the elevated temperature inside the reactor 10. The evaporated oil consisting primarily of long-chained hydrocarbons. These long-chained hydrocarbons are cracked to shorter-chained hydrocarbons. Un-cracked oil vapor continues to higher parts of the reactor 10. The short-chained hydrocarbons are used for NOx conversion. The cracking takes place on the outside of the catalyst, and the short-chained hydrocarbons penetrate the catalyst and the NOx conversion can take place.

Tests have shown that introducing an oxidation catalyst and a particulate filter makes it possible to achieve a heat recovery in the range of up to 100% of the lower heat value in the diesel oil injected. If the heat can be recovered and used on board on a ship in case the selective catalytic reduction system 2 is installed on a ship. Accordingly, there is no extra cost for reactant and no extra production of CO₂.

The NOx conversion process carried out in the selective catalytic reduction system 2 contains several steps, including:

Cracking

C₁₂H₂₃⇒C₃H₆+4C₂H₄+H⁺+C  (1)

NO Oxidation

NO+½O₂⇒NO₂  (2)

NO+NO₂+2H⁺⇒2NO++H₂O  (3)

Heterogeneous Catalyzing

4C₂H₄+10O₂+2NO+⇒N₂+2CO+6CO₂+8H₂O  (4)

C₃H₆+NO++2.5O₂⇒NH₄++CO+2CO₂+H₂O  (5)

Homogeneous Process:

NH₄++NO₂⇒N₂+2H₂O  (6)

When injecting diesel oil into the exhaust, having a temperature of at least 350° C., the diesel oil will be at least partly evaporated. The first step is the step of cracking of diesel oil to ethylene and propylene and other connections as defined by equation (1). Equations (4) and (5) illustrates that ethylene and propylene is oxidized and that N₂, CO and CO₂ is formed. This implies that heat is created.

Since the heating value of short-chained HC-connections is larger, than the heating value of long-chained HC-connections (see Table A below). The heating value of the cracked connections can be calculated by using equation (1):

42.8 MJ/kg⇒[(42/167)·45.66+(112/167)·47.74+(1/167)·120.1+12/167·29.5]MJ/kg  (7)

42.8 MJ/kg⇒46.3 MJ/kg (with the oxidation catalyst 136 and particulate filter 138)  (8)

42.8 MJ/kg⇒44.2 MJ/kg(with the oxidation catalyst 136)  (9)

TABLE A Diesel oil Propylene Ethylene Hydrogen Carbon Heating values [MJ/kg] 42.8 45.7 47.7 120 29.5

From equation (8) it can be seen that the heating value (when applying the oxidation catalyst 136 and particulate filter 138) is increased to 46.3 MJ/kg, which is larger than the heating value of (42.8 MJ/kg) of diesel oil. From equation (9) it can be seen that the heating value (when applying the oxidation catalyst 136) is increased to 44.2 MJ/kg, which is larger than the heating value of (42.8 MJ/kg) of diesel oil. Accordingly, by introducing an oxidation catalyst 136 and a particulate filter 138 it is possible to an increased heat production. The cracking process is essential and required to achieve this increased heat production.

FIG. 14A illustrates a schematic view of a selective catalytic reduction system 2 according to embodiments of the invention configured to carry out NOx reduction and produce steam and power. The selective catalytic reduction system 2 comprises the features of the system shown in FIG. 5B (these are the ones inside the box having dotted lines). A boiler 188 is connected to the reactor 10 and received exhaust 190 therefrom. The boiler 188 releases exhaust 190 through an exhaust outlet and releases steam 210 through a steam outlet that is connected to a steam turbine 230 that drives a generator 144 arranged and configured to produce power (electricity)

FIG. 14B illustrates a schematic view of a selective catalytic reduction system 2 configured to carry out NOx reduction and produce drinking water. The selective catalytic reduction system 2 comprises the features of the system shown in FIG. 2B (these are the ones inside the box having dotted lines). A boiler 188 is connected to the reactor 10 and received exhaust 190 therefrom. The boiler 188 releases exhaust 190 through an exhaust outlet and releases steam 210 through a steam outlet that is connected to a distillation apparatus 150 arranged and configured to distillate feed water (e.g. seawater) through a feed water inlet 174 and deliver drinking water through a drinking water outlet 172

FIG. 15 illustrates a schematic view of a selective catalytic reduction system 2 configured to carry out NOx reduction and produce steam and power. The selective catalytic reduction system 2 comprises the features of the system shown in FIG. 2A (these are the ones inside the box having dotted lines). A boiler 188 is connected to the exhaust turbine (expander) 82. The boiler 188 releases exhaust 190 through an exhaust outlet and delivers steam 210 to a steam turbine arranged and configured to drive a generator 144 configured to produce power (electricity). The generator 144 is also connected to a gas turbine being in fluid communication with the gas receiver 78 and the reactor 10.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.

LIST OF REFERENCE NUMERALS

-   2 Selective catalytic reduction system -   4 Diesel engine -   6 Oil injection system -   10 Reactor -   11, 11′, 11″ Selective catalytic reduction catalyst -   12, 12′, 12″ Selective catalytic reduction catalyst -   14 First section -   16, 16′ Additional section -   18, 18′, 18″ Door -   20, 20′ Conical portion -   22, 22′ End pipe -   24 Injection unit -   26, 26′ Pipe -   28 Temperature sensor -   30 Temperature sensor -   31 Pressure tank -   32 Differential pressure sensor -   33, 33′ Valve -   34, 34′ Conduit -   36 Pipe -   38 Cable -   40 Oil -   42 Cable -   44 Control valve -   46 Flow sensor -   48 Pump -   50 Diesel tank -   52, 52′, 52″ Cable -   54, 54′, 54″ Pipe -   55 Pipe -   56 Cable -   58, 58′ Flange -   60 Programmable Logic Controller (PLC) -   62 Control unit -   64 Air cooler -   66 Cylinder -   68, 68′, 68″, 68′″ Valve -   70 Piston -   72 Blower -   74 Control valve -   76 Scavenge air receiver -   78 Exhaust gas receiver -   80 Compressor -   82 Exhaust turbine (expander) -   84, 84′ Plate member -   86 Tubular structure -   88 Diffuser -   90, 90′ Support leg -   92 First end -   94 Second end -   96, 96′, 96″, 96′″ Connecting pipe -   98 Flange -   100 Manifold -   102 Flange -   104 Flange -   106 Nozzle -   108, 108 Substrate -   109, 109′ Catalyst -   110 Cracked diesel oil -   112, 112′ Passage -   114 Graph -   116 Residence time between the catalyst layers -   118 Relative activity (two layers relative to one layer) -   120 Point -   122 Point -   124 Number of layers -   126 Activity (NOx reduction [ppm]) -   134, 134′, 134″ Closing structure -   136 Oxidation catalysts -   138 Particulate filter -   140 Cubic cassette -   144 Generator -   150 Distillation apparatus -   172 Drinking water outlet -   174 Feed water (seawater) inlet -   188 Boiler -   190 Exhaust gas -   210 Steam -   230 Steam turbine -   232 Gas turbine -   D₁, D₂, D₃ Distance -   d₁, d₂, d₃, d₄ Distance -   ΔT₁, ΔT₂, ΔT₃ Temperature increase -   ΔT₄, ΔT₅ Temperature increase -   S₁, S₂, S₃, S₄, S₅ Layer of selective catalytic reduction catalysts -   S₁′, S₂′, S₃′, S₄′, S₅′ Layer of selective catalytic reduction     catalysts -   S₁″, S₂″, S₃″ Layer of selective catalytic reduction catalysts -   S₄″, S₅″, E₁, E₂ Layer of selective catalytic reduction catalysts -   L₁, L₂, L₃, L₄, L₅ Thickness -   h Distance -   H Height -   W Arrow -   W_(Q) Arrow -   N Number of layers 

1-24. (canceled)
 25. A selective catalytic reduction system applying diesel oil as reductant for converting nitrogen oxides in diesel engine exhaust, through a NO reduction process by a catalyst, into diatomic nitrogen and water in a diesel engine, wherein the selective catalytic reduction system is configured to be installed and used in a two-stroke diesel engine or a four-stroke diesel engine for marine purposes, wherein the selective catalytic reduction system comprises: an oil injection system; a reactor; a number of selective catalytic reduction catalysts provided in a section comprising a plurality of layers provided in a non-zero distance from each other, wherein the selective catalytic reduction system comprises a number of selective catalytic reduction catalysts provided in separated layers in a first section, wherein the selective catalytic reduction system comprises at least one additional section comprising a number of selective catalytic reduction catalysts in separated layers, wherein the at least one additional section is provided in a non-zero distance from the first section, wherein the reactor is configured to house the sections in order to facilitate the NO reduction process, wherein the distance between the layers is selected to ensure that a minimum average residence time for the exhaust is provided between each layer, wherein the minimum average residence time for moving a gas molecule from one layer to the next layer is no smaller than 0.025 seconds.
 26. A selective catalytic reduction system according to claim 25, wherein the minimum average residence time for moving a gas molecule from one layer to the next layer is no smaller than 0.04 seconds.
 27. A selective catalytic reduction system according to claim 25, wherein the minimum average residence time for moving a gas molecule from one layer to the next layer is no smaller than 0.135 seconds.
 28. A selective catalytic reduction system according to claim 25, wherein a plate member is provided between at least some of the adjacent sections.
 29. A selective catalytic reduction system according to claim 25, wherein the selective catalytic reduction catalysts are arranged in the layers each having a thickness within the range 40-120 mm, or in the range 50-100 mm, or in the range 60-90 mm.
 30. A selective catalytic reduction system according to claim 25, wherein the selective catalytic reduction catalysts are selected among the following: Ce/Cu-ZSM-5, Ce—Zr/Cu-ZSM-5, Ce/Fe-ZSM-5 or Ce—Zr/Fe-ZSM-5, Ce/Mg-ZSM-5 or Ce—Zr/Mg-ZSM-5.
 31. A selective catalytic reduction system according to claim 25, wherein the selective catalytic reduction system comprises a tubular structure extending centrally along the longitudinal axis of the reactor.
 32. A selective catalytic reduction system according to claim 25, wherein the selective catalytic reduction system comprises a diffuser.
 33. A selective catalytic reduction system according to claim 25, wherein the selective catalytic reduction system comprises a heat recovering unit.
 34. A method for nitrogen oxides reduction in diesel engine exhaust, said method comprising the step of using a selective catalytic reduction system configured to be applied for selective catalytic reduction of NOx in an exhaust stream either after a four-stroke diesel engine or between the exhaust receiver and the exhaust turbine(s) on a two-stroke diesel engine and applying diesel oil as reductant for converting nitrogen oxides through a NO reduction process by a catalyst into diatomic nitrogen and water in a diesel engine, wherein the method comprises the step of providing evaporated diesel oil in a reactor, wherein a NOx conversion is carried out including a NO reduction followed by a heterogeneous catalyzing process followed by a homogeneous catalyzing process by applying a number of selective catalytic reduction catalysts provided in a section having a plurality of layers, and further that the selective catalytic reduction system comprises a number of selective catalytic reduction catalysts provided in separated layers in a first section, wherein the selective catalytic reduction system comprises at least one additional section comprising a number of selective catalytic reduction catalysts in separated layers, wherein the at least one additional section is provided in a non-zero distance from the first section, wherein the reactor is configured to house the sections in order to facilitate the NO reduction process, wherein the distance between the layers is selected to ensure that a minimum average residence time for the exhaust is provided between each layer, wherein the minimum average residence time for moving a gas molecule from one layer to a next layer is no smaller than 0.025 seconds.
 35. A method according to claim 34, wherein the minimum average residence time for moving a gas molecule from one layer to a next layer is no smaller than 0.04 seconds.
 36. A method according to claim 34, wherein the minimum average residence time for moving a gas molecule from one layer to a next layer is no smaller than 0.135 seconds.
 37. A method according to claim 34, wherein the method comprises the step of cracking at least a portion of the evaporated diesel oil, carrying out a heterogeneous NOx conversion that causes formation of the radical NH4⁺, wherein the radical NH4⁺ is used for carrying out homogeneous NO_(x) conversion.
 38. A method according to claim 34, wherein the method is applied for selective catalytic reduction of NOx in an exhaust stream either after a four-stroke diesel engine or between the exhaust receiver and the exhaust turbine(s) on a two-stroke diesel engine.
 39. A selective catalytic reduction system according to claim 25 wherein an oxidation catalyst arranged to oxidise one or more compositions in the exhaust gas.
 40. A selective catalytic reduction system according to claim 39, wherein the selective catalytic reduction system comprises a flow-through type diesel oxidation catalyst and a soot particle filter.
 41. A selective catalytic reduction system according to claim 39, wherein the oxidation catalyst is a wall flow type particle filter comprising walls coated with a catalyst configured to oxide at least CO or HC.
 42. A selective catalytic reduction system according to claim 39, wherein the oxidation catalyst is provided downstream the selective catalytic reduction catalysts and a particulate filter.
 43. A selective catalytic reduction system according to claim 39, wherein the selective catalytic reduction system comprises or is connected to a generator.
 44. A method according to claim 34 wherein the method comprises the step of applying an oxidation catalyst arranged to oxidise one or more compositions in the exhaust gas.
 45. A method according to claim 44, wherein the method comprises the step of applying a wall flow type particle filter comprising walls coated with a catalyst configured to oxide at least CO or HC.
 46. A method according to claim 44, wherein the method comprises the step of applying a flow through type diesel oxidation catalyst and a particle filter.
 47. A method according to claim 44, wherein the method comprises the step of applying an oxidation catalyst provided between the selective catalytic reduction catalysts and a particulate filter.
 48. A method according to claim 44, wherein the method comprises the step of applying a generator connected to a turbine to produce electrical energy.
 49. A method according to claim 44, wherein the method comprises the step of applying a distillation apparatus to produce distilled water, wherein the distillation apparatus is connected to the selective catalytic reduction system. 